1. Field of the Invention
The present invention relates to gas sensors.
2. Description of the Related Art
To date, use of urea as a reducing agent is known as a method for purging NOx from exhaust gas from diesel engines. For example, PTL 1 describes a configuration in which an injector, which injects urea into an exhaust pipe, a selective catalytic reduction (SCR) catalyst, which reduces nitrogen oxide (NOx) using ammonia produced at the time of hydrolysis of urea so that Nox is converted into harmless N2 and H2O, and an ammonia-concentration sensor, which detects an excessive ammonia concentration contained in the exhaust gas that has passed the SCR catalyst, are disposed on an engine exhaust path. PTL 1 also describes that the rate of urea injected into the exhaust pipe from the injector is controlled so that the ammonia concentration detected by the ammonia-concentration sensor becomes closer to zero. The ammonia-concentration sensor includes a sensor element, formed by stacking multiple oxygen-ion-conducting solid electrolyte layers one on top of another, and a protective cover, which controls flow of gas to the sensor element or prevents water from adhering to the sensor element. PTL 1 describes that coating the protective cover with a coating layer prevents ammonia from being decomposed by the protective cover so that degradation of the ammonia-concentration detection sensitivity is suppressed.